By way of background, compression molding is a technique for forming parts wherein a charge is placed between upper and lower heated die members defining a mold cavity. The dies are then brought to a closed position where the dies compress the charge causing it to flow and fill the mold cavity. After the resin cures, the molds are opened and the finished part removed.
Compression molding techniques have been used to make parts having a relatively flat surface, such as exterior automotive body panels. The charges used for making such parts generally consist of a thermosetting resin containing reinforcing fibers and various fillers. Quite often, the charges are formed into sheets known in the art as sheet molding compounds (SMC). Unfortunately, it has been extremely difficult to mold fiberglass reinforced plastic (FRP) parts so that they have an extremely smooth surface. Quite often, the molded parts have surfaces that are blistered, rough or porous. It is commonly believed that such defective areas result primarily from air that is trapped in the charge during molding. In an effort to minimize trapped air it is conventional practice to use a relatively thick charge which covers a relatively small area of the molding surface so that the air in the charge is "squeezed" out when the dies are closed.
It has been recognized that the use of vacuum during the compression molding process may be useful in reducing the number of defects in the surface of the part. (See, e.g. U.S. Pat. No. 3,840,239 to Fazekas et al. and U.S. Pat. No. 4,488,862 to Epel et al which is assigned to the assignee of the present invention.) However, the results were not always completely satisfactory. (See Gorsuch et al, "Surface Porosity and Smoothness of SMC Molding as Affected by Vacuum and Other Molding Variables", 33rd Annual Technical Conference, 1978, Reinforced Plastics/Composites Institute, the Society of the Plastics Industry, Inc., Section 9-F, pages 1-7).
In more recent years the industry has used what is known as an "in-mold coating" technique to provide parts with a commercially acceptable smooth finish. This "in-mold coating" technique is disclosed, for example, in U.S. Pat. No. 4,081,578 to van Essen et al. Briefly, this method employs an additional processing step where the cured part remains in the mold and is coated with a composition that spreads and penetrates the surface to fill the pores and voids therein.
Unfortunately, this technique has several drawbacks. For example, the additional coating operation consumes valuable machine time and lessens the amount of production which can be gained from a single mold. Relatively sophisticated and expensive mechanisms must be utilized to control the application of the coating to the part surface, and care must be taken to insure that the coating will properly bond to the surface of the part. Those skilled in the art are probably aware of still other problems that are associated with the use of the "in-mold coating" process. However, despite these problems, it is generally accepted in the industry that the "in-mold coating" process is required in order to provide commercially acceptable parts, at least for exterior automotive body panels.